1. Field of the Invention
The present invention relates to a charging device for use in a copier, a printer or like image forming apparatus that utilizes electrophotographic technologies.
2. Description of the Prior Art
In conventional image forming apparatus employing the electrophotographic technology (Carlson Process), a corona charging device utilizing corona discharges has been used to charge a photosensitive body to a certain potential. However, this type of a charging device often generates electrical noises in peripheral devices due to a high voltage required for corona discharge, or discomforts persons around the apparatus because of much ozone generated from the device. In view of those disadvantages, another charging device has been proposed to replace the corona charging device, in which a roller or strip-shaped charging member provided with conductive yarns or conductive resin over its surface contacts with a photosensitive body to be charged, and a voltage is applied across the charging member and the photosensitive body. This charging device has enabled charging without using a high voltage. It electrically feeds the photosensitive body via a contact portion therebetween and utilizes discharge generated across a small gap adjacent the contact portion.
The charging principle for this charging device will be described with reference to FIGS. 1 and 2. In the charging device, a conductive brush roller 5 provided with conductive yarns 5a on its surface is used as a charging member. The charging brush roller 5 and a photosensitive body 1 contact with each other at a contact point B and rotate in opposite directions with each other. As described below, across the conductive brush roller 5 and the photosensitive body are applied d.c. voltages only. Let an arbitrary point A on the photosensitive body 1 on the verge of rotating be within a certain distance from a conductive yarn 5a as shown in FIG. 2. If an applied voltage V.sub.ap is greater than a discharge starting voltage V.sub.th determined by the above-mentioned distance, discharge from the conductive yarn 5a occur, whereby the charging of the photosensitive body 1 is initiated. Then, the charged voltage denoted as V.sub.sp on the photosensitive body 1 rises with the discharge from the conductive yarn 5a until the difference between the applied voltage and the supplied charge on the photosensitive body 1 becomes equal to the discharge starting voltage, and then the discharge stops. Further, the d.c current flowing through the conductive yarn 5a produces a voltage drop denoted as V.sub.down on the conductive yarn 5a. Therefore, supposing dark decay on the photosensitive body 1 is negligible, the potential on the conductive yarn 5a can be indicated by the following equation: EQU V.sub.sp =V.sub.ap -V.sub.th -V.sub.down ( 1)
Owing to the rotation of the photosensitive body 1, the point A passes through the discharge area C and reaches the point B with the supplied charge maintained thereon. At the contact point B, the point A is charged by the conductive yarn 5a, whereby its potential further rises. The final potential denoted as V.sub.sp at the point A can be indicated by the following equation: EQU V.sub.sp =V.sub.ap -V.sub.th -V.sub.down +V.sub.inj.DC1 ( 2)
where V.sub.inj.DC1 is a voltage supplied by the charge injection through the contact point. It will be appreciated from the equation (2) that the supplied charge on the photosensitive body 1 is the sum of the potential rise by the discharge from the conductive yarn 5a and the charge supplied through the contact portion with the conductive yarn 5a. The amount of injected charge is determined by the contact resistance which varies depending on the condition of the surfaces of the contacting members. That is, under high humidity, moisture adheres to the contact point. Therefore, the electric resistance at the contact point decreases, whereby the amount of the injected charge increases on the photosensitive body 1. Further, the condition of the contact point and the electric resistance thereof gradually varies with the passing of time, which varies the amount of supplied charge. Therefore, it is difficult to realize a stable potential on the photosensitive body 1 by the above prior art method. Accordingly, the method using the prior art charging device is not applicable to a practical use.
The Japanese Unexamined Patent Publication No. 46265/1982 concerns a charging device according to the electorophotographic technology, but it does not teach or suggest specifying the hygroscopic degree of yarns provided on the charging device. The Japanese Unexamined Patent Publication No. 2312/1993 also concerns a like charging device, but its teaching about the hygroscopic degree of yarns is 10% or less and further the resin to be used for making the yarns is limited to polyamide (nylon).